WO 2006/073034A1 (Patent Document 1) and WO 2006/077714A1 (Patent Document 2) each disclose an antenna that operates in a plurality of frequency bands.
Here, a configuration of the antenna disclosed in Patent Document 1 will be described with reference to FIG. 1. In the example of FIG. 1, a feed radiation electrode 7 is formed on a prism-shaped dielectric base 6. The feed radiation electrode 7 resonates in a fundamental mode and in a higher-order mode, and one end of the feed radiation electrode 7 defines a feed end 7A connected to a circuit for radio communication. The other end 7B of the feed radiation electrode defines an open end. The position of a capacitance-loading portion α is set in advance between the feed end 7A and the open end 7B of the feed radiation electrode 7, and a capacitance-loading conductor 12 is connected to the capacitance-loading portion α. The capacitance-loading conductor 12 forms a capacitance for adjusting a resonant frequency in the fundamental mode, between the feed end 7A and the capacitance-loading portion α.
Further, in the antenna disclosed in Patent Document 2, a feed radiation electrode and a non-feed radiation electrode, each having a spiral slit, are formed on a dielectric base. The dielectric base is provided in a non-ground area of a substrate, and a capacitance occurs at each of the spiral slits.
According to the antenna disclosed in Patent Document 1, the magnitude of the capacitance connected between the feed end 7A and the capacitance-loading portion α is set by the capacitance-loading conductor 12. Thus, the resonant frequency in the fundamental mode can be adjusted. In addition, by appropriately setting the position of the capacitance-loading portion α, the resonant frequency in the fundamental mode can be adjusted with a resonant frequency in a harmonic mode being maintained constant.
However, the shape of an electrode pattern on the prism-shaped dielectric base needs to be changed for adjusting or changing the loaded capacitance. The same is true for the antenna disclosed in Patent Document 2. For example, when the antenna is caused to operate as an antenna for two frequency bands, the 2 GHz band and the 900 MHz band, the resonant frequency in the fundamental mode is set at the 900 MHz band, and the resonant frequency in the harmonic mode is set at the 2 GHz band. When the resonant frequency in the harmonic mode is changed, and when the resonant frequency in the fundamental mode is changed by the loaded capacitance as well, the electrode pattern has to be changed. Thus, there is a problem that a period for development and designing is needed, resulting in an increase in cost.